The challenges of laser cutting: Overcoming some common obstacles

THE FABRICATOR® AUGUST 1998

March 13, 2002

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Lasers can be used to process expensive alloys as well as traditional materials such as stainless steel. However, knowing the strengths and weaknesses of laser processing is the key to determining whether or not a laser is the right choice for cutting.

While lasers are not the answer for every application, they can provide flexibility, efficient material use, and a repeatable, controlled process.

One advantage that lasers have over conventional processes such as stamping and punching is that they work with minimal contact. A typical cut width of .010 inch enables lasers to be used for small-radius cutting. This small kerf allows close nesting of parts and helps to minimize material waste. In addition, materials may be heat treated after cutting without the distortion that can occur with the grinding and reforming usually needed after processing by other methods.

Material Considerations

Laser cutting becomes less effective when material thickness increases. For instance, lasers may be ineffective for cutting carbon steel thicker than .4 inches. Laser cutting thickness limitations are determined by heat conductivity, surface reflection at 10.6 microns, the vaporization point of alloys, the types of alloys, surface tension of molten materials, and part geometry. As thickness increases, the likelihood of a blowout or thermal runaway also increases.

By highly focusing its beam and reducing its spot size, the laser becomes a sharper cutting tool. For example, a laser's cutting ability increases significantly when the spot size is narrowed from even .004 to .011 inches in diameter. Ideally, the wavelength will allow 100 percent absorbability with the material.

In many instances, the laser's ability to cut can be further improved by focussing the assist gas. The assist gas serves two purposes: to help in combustion, and to blow the debris or molten metal away from the kerf.

Other conditions contribute to processing problems. If the thermal conductivity is high in a material such as aluminum, much of the energy is transferred laterally into the material, which results in inefficient cutting and reduced cutting speeds. As a general rule, if energy is transferred into a material inefficiently, the effectiveness of the cutting process is reduced.

Certain amounts of metal become vapor or molten during the cutting process. The laser acts more like a plasma cutter when thicker materials are processed and larger amounts of material become molten.

Part Geometry. Certain part geometries are affected more than others by the thermal process. For instance, corner or smaller areas of a part absorb more heat, and consequently the probability of thermal runaways or violent reactions like blowouts increases.

Generally, the more complicated the part geometry, the more difficult it is to maintain constant cutting speeds. Often, speed and productivity are compromised when cutting shapes with varying curves and angles. It is generally more efficient to speed up a laser when cutting curves to prevent overheating the part and deteriorating edge quality.

Pulsing the laser rather than using a continuous wave to pop or drill holes is one method that is used for avoiding thermal problems.

Material Composition & Quality

Material Composition. Material composition affects laser processing more dramatically than it does other processing methods. Laser processing is influenced by conductivity and the viscosity of metal in a liquid state.

The surface tension of liquid metal affects the degree of dross that adheres to the exit edge of the part. If the viscosity is thin, the dross will blow away. However, if it is thick, the dross will cling to the material and usually elevate the temperature of the part. In such cases, secondary operations may be required to remove the recast.

Carbon content assists in combustion, making higher-carbon steels—materials that are of higher quality and used as structural steels—combust quickly when they come into contact with the laser beam. Compared to more homogenous materials, carbon steels may contain a wide range of elements that have different melting points. During the thermal process, random reactions can occur because of element variance and the surface condition of the material. These surface conditions include scale, coatings, filth, and surface impurities.

The dwelling process is sometimes used to cut particularly reflective material like aluminum. A laser can dwell in the start position until the material reflectivity changes and before the motion system begins to move the laser.

Surface tension, carbon content, and the absorbability of material at 10.6 microns also affect laser performance. For example, because of its reflectivity, copper is very difficult to cut with a laser, but not with other cutting methods.

Material Quality. Laser processing is more sensitive to material quality than are other processes. The surface finish can dramatically affect the quality of cutting. In most cases, steel must be clean, pickled, and oil-free. Impurities on low-grade steel are highly reactive to the thermal process, especially when oxygen is used as a processing gas.

Hot rolled steel presents serious quality problems in cutting because of the surface scale. The surface tends to melt in with the metal, creating an undesirable finish. If the material surface is not smooth, the assist gas and laser focus can be altered, affecting the quality of the cut.

Laser cutting can leave a recast layer on the surface. Because lasers melt and burn some of the metal, remelted materials are deposited on the side of the cut edges and on the bottom of the cut. This layer of deposited materials is highly stressed and may crack, especially if it is an oxide. Although these cracks are small, they can propagate into the material, creating larger cracks. This is especially true of inside corners with small radii, where stresses are higher.

Cracks can be eliminated in certain cases with high-pressure nitrogen cutting. For example, titanium is extremely reactive to oxygen. Oxygen embrittlement can lead to micro-cracks, which are an important safety concern for users of titanium such as the aerospace and medical industries, in which long-term strength and avoiding fatigue cracks is critical. Using high-pressure nitrogen is often the solution. This method effectively turns the laser into a controlled welding machine, blowing molten material away with 160 pounds per square inch (PSI) gas.

High-pressure nitrogen cutting is more expensive and time-consuming because its motion system must be very precisely controlled. However, high-pressure nitrogen cutting offers very high cutting speeds. Currently, this method of cutting is used on 300 series stainless steels.

Setup Time

Setup time for a laser can be lengthy when preparing to cut unfamiliar materials. Nozzle size, power, optical focal length, assist gas, gas pressure, speed, and focal length can all influence the process. These parameters are so important to the process that if they are not set correctly, the material cannot be cut. However, when the combinations are correct, the speed of cutting can be several times faster than that of other cutting methods.

Several methods can be used to reduce setup time. Keeping meticulous records is a must. Using a reliable database of time-tested parameters of known materials, specifications, and conditions is critical in saving setup time.

Parameters are often wide-ranging for the same material. Requirements for surface finish, tolerances, heat-affected zone (HAZ), and flatness can significantly change the parameters.

Setup time can be greatly reduced by planning similar jobs to run together, creating modular fixtures, building ergonomic workstations, using shuttle tables, and simply keeping the material near the laser cutting table. Efficiency can also be increased by using delivery optics that can be quickly changed, eliminating alignment and indicating procedures.

Distortion

A heat-affected zone (HAZ) is produced during laser cutting. A HAZ forms in metals when the temperature rises above the critical transformation point. In laser cutting, this is localized near the cutting zone. In carbon steel, the higher the hardenability, the greater the HAZ.

For example, laser processing produces a HAZ of about .18 millimeter on 7-millimeter-thick, 4140 steel. Since the HAZ is brittle, this area has a lower tolerance for cracking during bending or stress. In most cases, the HAZ can be eliminated by post-heat treating the part, but there is a risk of distortion.

Laser cutting creates more stresses in material than do other methods such as waterjet cutting. In most cases, laser processing produces little distortion in material, but this depends on the laser's parameters, the material thickness, and its composition.

Distortion is more likely to occur when a laser is applied to thin materials with wall thicknesses of .001 to .005 inches or shim-stock material. Thinner materials are more easily distorted because a recast layer forms on the edge, and the resolidification of the molten material can more easily warp thinner material.

Distortion from laser processing is a result of the sudden rise in temperature of the material near the cutting zone. Distortion is also created by the rapid solidification of the cutting zone. In addition, distortion also can be attributed to the rapid solidification of material remaining on the sides of the cut. Adding a water quenching system to the laser cutting nozzle can reduce heat-induced stress.

Misting the part will lower the overall temperature, but will not affect the recast layer. This process works well when cutting tubes with diameters less than 1/4 inch and usually prevents slag from forming on the opposing walls.

Drilling

Drilling starting holes is faster with a laser but not as safe as other methods. While lasers can drill holes very quickly, a blowout may occur during the drilling process. Blowouts are more likely to take place if the hole diameter is small in relation to the material thickness.

Using a continuous-wave laser creates a high probability of a blowout when piercing metal. Many times, the solution calls for changing parameters so that the pierce is completed by pulsing the laser. The rest of the cutting can be done with a continuous wave.

To reduce the chances of a blowout occurring, the gas pressure is lowered to a level that still allows combustion. Controlling the focal point of the laser and cleaning the surface of the material to remove oil and grime are also important in preventing blowouts.



Jim Wollenberger

Contributing Writer

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